Methods and apparatus for wireless communication using time division duplex time-slotted CDMA

ABSTRACT

Methods and apparatus for wireless communication in systems such as omni-beam and narrow-beam fixed wireless loop (FWL) systems. In a first technique in accordance with the invention, referred to as code division duplex (CDD) time-slotted CDMA, uplink and downlink portions of the system are separated using code division duplexing, while the users within a given cell are also separated using codes, e.g., using time-slotted CDMA. In a second technique in accordance with the invention, referred to as time division duplex (TDD) time-slotted CDMA, uplink and downlink portions of the system are separated using time division duplexing, e.g., time slots, while the users in a given cell are separated using codes, e.g., time-slotted CDMA. Both the CDD and TDD techniques may make use of an electronically-steered beam which is designed to provide simultaneous coverage within a given cell for two or more users separated by codes. In a third technique in accordance with the invention, referred to as orthogonal frequency division multiplexing (OFDM), uplink and downlink portions of the system are separated in frequency, while the users are, e.g., also separated in frequency.

RELATED APPLICATIONS

The present application is related to the following U.S. patentapplications, both filed concurrently herewith in the name of inventorSyed Aon Mujtaba: U.S. patent application Ser. No. 09/200,523 entitled“Methods and Apparatus for Wireless Communication Using OrthogonalFrequency Division Multiplexing,” and U.S. patent application Ser. No.09/200,521 entitled “Methods and Apparatus for Wireless CommunicationUsing Code Division Duplex Time-Slotted CDMA.”

FIELD OF THE INVENTION

The present invention relates generally to communication systems, andmore particularly to wireless communication systems such as codedivision multiple access (CDMA) systems for fixed wireless loop (FWL)and other applications.

BACKGROUND OF THE INVENTION

FIG. 1 shows a portion of a conventional omni-beam FWL system 10. Theportion of system 10 shown includes four hexagonal cells 12-1, 12-2,12-3 and 12-4, each with a corresponding base station 14-1, 14-2, 14-3and 14-4, and a subscriber unit 16. The system 10 will generally includenumerous additional cells, base stations and subscriber units configuredin a similar manner. It is assumed in this system that the base stationsare equipped with omni-directional antennas, and that the positions ofthe subscriber units are fixed. The base station 14-3 of FIG. 1 is incommunication with the subscriber unit 16 in cell 12-3, e.g., forproviding a communication channel for an on-going voice or data call.The omni-beam FWL system 10 may be configured using a number ofdifferent techniques.

FIG. 2 shows an example of how the omni-beam FWL system 10 may beimplemented using a time division multiple access (TDMA) technique suchas that used in the Digital European Cordless Telephone (DECT) standard.In accordance with this TDMA technique, different frequencies are usedfor the different cells, such that among the cells, users are separatedin frequency. A suitable frequency reuse pattern, e.g., a seven-cellhexagonal reuse pattern, may also be used in order to limit the numberof different frequencies required. Within a given cell, users areseparated in time through the use of a sequence of time slots 20,including time slots 22-1, 22-2, . . . 22-N. The system 10 may also beimplemented using a code division multiple access (CDMA) technique. Inaccordance with this technique, the same frequencies but different codesare used for each of the cells, such that the codes are used to separateusers in different cells and within a given cell. Some frequencyseparation may also be used in conjunction with the code separation inorder to reduce interference from other cells. Additional detailsregarding conventional CDMA systems are described in, for example,Andrew J. Viterbi, “CDMA: Principles of Spread Spectrum Communication,”Addison-Wesley, 1995, which is incorporated by reference herein. Otherconventional CDMA systems are described in, for example, TIA/EIA/IS-95A,“Mobile Station—Base Station Compatibility Standard for Dual-ModeWideband Spread Spectrum Cellular System,” June 1996, and ANSIJ-STD-008, “Personal Station—Base Station Compatibility Requirements for1.8 to 2.0 GHz Code Division Multiple Access (CDMA) PersonalCommunication Systems,” both of which are incorporated by referenceherein.

FIG.3 shows a conventional narrow-beam FWL system 30. The portion ofsystem 30 shown includes four hexagonal cells 32-1, 32-2, 32-3 and 32-4,each with a corresponding base station 34-1, 34-2, 34-3 and 34-4. Inthis system, it is again assumed that the positions of the subscriberunits are fixed. The base stations in system 30 are equipped withdirectional antennas which generate narrow beams 36. At any given time,only a subset of the total number of beams in the system is active,i.e., communicating with users. The beams 36 are made as narrow aspossible in order to target only a single user, and thereby minimizinginter-cell interference. In order to provide an increased capacity, thesystem 30 may be configured such that all cells use the samefrequencies, i.e., a frequency reuse factor of 1. FIG. 4 shows analternative implementation in which a given cell 42-i includes nineelectronically-steerable narrow beams 46. The beams 46 are separatedinto three sectors, each including three beams designated 1,2 and 3.This provides a more manageable hopping pattern, e.g., turning on adesignated single beam within each sector at any given time.

FIGS. 5 and 6 illustrate the difference between sectorization andsteerable beams in a narrow-beam system such as system 30 of FIG. 3,which assumes a frequency reuse factor of 1. FIG. 5 shows a pair ofsectorized cells 50-1 and 50-2 having base stations 52-1 and 52-2,respectively. In this example, a beam 53 from one of six sectors of thecell 50-1 and a beam 55 from one of the six sectors of the cell 50-2will generate co-channel, i.e., inter-cell, interference. If the beamsare sectorized but not steerable, then it is generally not possible tomitigate this type of co-channel interference adaptively unless thesectors are separated in frequency. FIG. 6 shows an arrangement in whicha pair of cells 60-1 and 60-2, via respective base stations 62-1 and62-2, generate sectorized and steerable beams. It can be seen that, asillustrated by the relative positions of steerable beams 63 and 65, thatsuch an arrangement can be used to provide adaptive mitigation ofco-channel interference.

FIG. 7 illustrates a conventional technique for separating uplink (UL)and downlink (DL) traffic for a given antenna beam in an omni-beam ornarrow-beam system. In this technique, an uplink channel 72 _(U) and adownlink channel 72 _(D) are separated in frequency as shown, i.e.,frequency division duplexing (FDD) is used to separate uplink anddownlink traffic. Users of the uplink and downlink channels 72 _(U) and72 _(D) are separated in time, using sequences of time slots 74-1, 74-2,74-3 . . . and 76-1, 76-2, 76-3 . . . , respectively.

The conventional techniques described above suffer from a number ofdisadvantages. For example, it is generally very difficult to generatenarrow beams targeted to single users, as in the narrow-beam FWL system30 of FIG. 3. In addition, narrow beams of this type are susceptible toincreased interference from effects such as shadowing and problematicsidelobes. Use of narrow beams in conjunction with a TDMA techniquewithin a given cell can lead to catastrophic interference. For example,if beams from adjacent cells overlap, there is catastrophic interferencesince the signals are neither separated in frequency nor in time amongthe different cells, but are instead separated in the spatial domain. Ina high density environment, this limitation can severely restrictcapacity. Another problem is that conventional FDD techniques, such asthose used to separate uplink and downlink in FIG. 7, generally cannotadaptively tradeoff capacity between uplink and downlink. As a result,these FDD techniques are generally not well suited for use with, e.g.,data-oriented wireless services. It is apparent from the foregoing thatfurther improvements are needed in wireless communication techniques inorder to overcome these and other problems of the prior art.

SUMMARY OF THE INVENTION

The invention provides apparatus and methods for wireless communicationin fixed wireless loop (FWL) and other types of systems in which, e.g.,information is communicated in a given cell of the system betweensubscriber units and a base station over an uplink and a downlink. Inaccordance with a first aspect of the invention, a code division duplex(CDD) time-slotted CDMA wireless communication system is provided.Communications on the uplink are separated from communications on thedownlink using code division duplexing, and communications withdifferent subscriber units in the cell are separated using a codedivision multiple access technique, e.g., time-slotted CDMA. The codedivision duplexing may be implemented by, e.g., assigning a first subsetof a set of codes to the uplink and a second subset of the set of codesto the downlink. The code assignment process may be repeated fordifferent time slots, such that the number of codes in the first andsecond subsets varies across the time slots in accordance with uplinkand downlink traffic demands. The system may utilizeelectronically-steered beams generated by antennas associated with thebase stations. Any particular beam at a given time may have a widthsufficient to provide simultaneous coverage for at least n of thesubscriber units at that time, where n is greater than or equal to two.The n subscriber units are assigned different codes as part of the codedivision multiple access technique.

In accordance with another aspect of the invention, a time divisionduplex (TDD) time-slotted CDMA wireless communication system isprovided. Communications on the uplink are separated from communicationson the downlink using time division duplexing, and communications withdifferent subscriber units in the cell are separated using a codedivision multiple access technique, e.g., time-slotted CDMA. The timedivision duplexing may be implemented by, e.g., assigning a first subsetof a set of time slots to the uplink and a second subset of the set oftime slots to the downlink. The time slot assignment process may beimplemented such that the assignment of time slots to uplink anddownlink is varied in accordance with uplink and downlink trafficdemands. A TDD time-slotted CDMA system in accordance with the inventionmay also make use of the above-noted electronically-steered beams, eachhaving a width sufficient to provide simultaneous coverage for at leastn subscriber units at a given time.

In accordance with another aspect of the invention, an orthogonalfrequency division multiplexing (OFDM) wireless communication system isprovided. Communications on the uplink are separated from communicationson the downlink using OFDM. Subscriber units in the cell are separatedusing, e.g., code division multiple access, time division multipleaccess, frequency division multiple access or combinations of these andother techniques. The OFDM may involve, e.g., assigning a first subsetof M OFDM carriers to the uplink and a second subset of the M carriersto the downlink. The carrier assignment process may be repeated fordifferent time slots, such that the number of carriers in the first andsecond subsets varies across the time slots in accordance with uplinkand downlink traffic demands.

The invention provides improved performance in wireless communicationsystems, particularly in applications involving heterogeneous traffic,e.g., mixed voice and data traffic, and other applications in whichuplink and downlink capacity requirements are subject to largefluctuations. The invention is particularly well suited for use inapplications such as omni-beam and narrow-beam FWL systems, although itcan provide similar advantages in numerous other wireless communicationapplications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a portion of a conventional omni-beam FWL system.

FIG. 2 illustrates a conventional TDMA technique for use in the FWLsystem of FIG. 1.

FIG. 3 shows a portion of a conventional narrow-beam FWL system.

FIG. 4 illustrates an example of sectorization in a narrow-beam FWLsystem.

FIGS. 5 and 6 illustrate distinctions between conventional sectorizedand steerable beams.

FIG. 7 illustrates a conventional technique which utilizes frequencydivision duplexing (FDD) to separate uplink and downlink and a TDMAtechnique to separate users.

FIGS. 8 and 9 illustrate a code division duplex (CDD) time-slotted CDMAtechnique in accordance with the invention.

FIG. 10 illustrates a time division duplex (TDD) time-slotted CDMAtechnique in accordance with the invention.

FIG. 11 illustrates an orthogonal frequency division multiplexing (OFDM)technique in accordance with the invention.

FIGS. 12 and 13 show a downlink transmitter and a downlink receiver,respectively, for implementing the OFDM technique of FIG. 11.

FIGS. 14 and 15 show an uplink transmitter and an uplink receiver,respectively, for implementing the OFDM technique of FIG. 11.

FIGS. 16 and 17 show a multi-code CDMA transmitter and a multi-code CDMAreceiver, respectively, in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be illustrated below in conjunction withexemplary wireless communication systems and communication techniques.It should be understood, however, that the invention is not limited touse with any particular type of communication system, but is insteadmore generally applicable to any wireless system in which it isdesirable to provide improved performance without unduly increasingsystem complexity. For example, it will be apparent to those skilled inthe art that the techniques are applicable to omni-beam and narrow-beamfixed wireless loop (FWL) systems, CDMA systems, as well as to othertypes of wideband and narrowband wireless systems. The term “subscriberunit” as used herein is intended to include fixed terminals such asfixed wireless installations, mobile terminals such as cellulartelephones and portable computers, as well as other types of systemterminals. The term “separating” as applied, e.g., to uplink anddownlink or subscriber units in a given cell of a system, refersgenerally to implementing the system such that interference between,e.g., the uplink and downlink or the subscriber units, is reduced,minimized, or eliminated.

The invention provides a number of communication techniques forovercoming the above-noted problems of the prior art. The techniquesdiffer in terms of the manner in which uplink and downlink portions ofthe system are separated, and/or the manner in which users are separatedwithin a given cell. As noted previously, conventional techniquesgenerally separate uplink and downlink portions of the system usingfrequency, e.g., FDD as shown in FIG. 7, and separate users within agiven cell using, e.g., time slots as shown in FIG. 7 or codes. In afirst technique in accordance with the invention, referred to herein ascode division duplex (CDD) time-slotted CDMA, uplink and downlinkportions of the system are separated using codes, while the users arealso separated using codes. In a second technique in accordance with theinvention, referred to herein as time division duplex (TDD) time-slottedCDMA, uplink and downlink portions of the system are separated usingtime slots, while the users are separated using codes. In a thirdtechnique in accordance with the invention, referred to herein asorthogonal frequency division multiplexing (OFDM), uplink and downlinkportions of the system are separated in frequency, while the users arealso separated in frequency. Each of these techniques will be describedin greater detail below.

An illustrative embodiment of the CDD time-slotted CDMA technique of theinvention will be described with reference to FIGS. 8 and 9. FIG. 8shows a single cell 80-1 of a wireless system. The cell includes a basestation 82-1 and a number of subscriber units 84. As shown, a singleantenna beam 86 generated by the base station 82-1 is directed toseveral subscriber units, i.e., five subscriber units in this example.The beam 86 is approximately 40° wide, such that there will be a totalof nine beams generated in each cell. The additional beams are omittedfrom FIG. 8 for clarity of illustration. It is also assumed that thebeams in the cell 80-1 and the other cells of the corresponding systemare electronically steerable. The beam 86 in FIG. 8 is purposely madewider than the typical single-user narrow beam in a conventional systemsuch as system 30 of FIG. 3, in order to target more than one subscriberunit. Although the beam 86 is broader than, e.g., the beam 63 or 65 inFIG. 6, it can be configured to span a smaller portion of its sector.Within a given cell, such as cell 80-1, users are separated by codes,i.e., assigned different codes to prevent the users in the beam 86 frominterfering with one another. Among adjacent cells, users are alsoseparated by codes. Thus, when beams from adjacent cells collide, theinterference will not be catastrophic since the users in adjacent cellsare separated by codes. Standard CDMA techniques, such as thosedescribed in the above-cited CDMA references, may be used to separatethe users within a cell and among adjacent cells. The technique is“time-slotted” in that the beams are steerable, such that differentbeams can be activated in different time slots, and may also be referredto as “discontinuous-transmission” CDMA.

FIG. 9 shows an exemplary CDD mechanism suitable for use in the CDDtime-slotted CDMA technique of the invention. In this embodiment, theCDD mechanism is implemented by using different codes for the uplink anddownlink portions of the system. For example, as shown, the uplink usescode N, code N−1, etc., while code 1, code 2, etc., are used for thedownlink. The boundary 90 between the uplink codes and the downlinkcodes is variable, such that the capacity allocated to uplink anddownlink can be adaptively altered to account for demand variations. Forexample, the boundary 90 can vary for each time slot, or for each groupof a predetermined number of time slots.

The CDD time-slotted CDMA technique described above provides a number ofadvantages over conventional techniques. For example, a systemimplemented using such a technique does not require an unduly narrowbeam designed to target a single subscriber unit. In addition, uplinkand downlink can be traded off by reassignment of uplink and downlinkcodes, and an efficient closed loop power control process can bemaintained since both the uplink and downlink can be on the samefrequency. A fixed quality of service (QoS) can be provided for a givenuser by utilizing the same uplink-downlink code boundary for each slotassigned to that user. Moreover, the variable boundary makes it easierto accommodate variable rate users, e.g., through multicode or variablerate spreading, and to transmit heterogeneous traffic, e.g., voice anddata traffic.

FIG. 10 illustrates a TDD time-slotted CDMA technique in accordance withthe invention. This technique is the same as the CDD time-slotted CDMAtechnique described in conjunction with FIGS. 8 and 9, except that adifferent duplexing mechanism, i.e., a time division rather than codedivision technique, is used to separate the uplink and downlink portionsof the system. FIG. 10 illustrates the duplexing used in the TDDtime-slotted CDMA technique. One or more of the time slots are assignedto the downlink, while others are assigned to the uplink. The assignmentof time slots to uplink or downlink may be varied adaptively, so as toaccommodate variations in uplink and downlink traffic demands. The otheraspects of the system are otherwise the same as in the CDD time-slottedCDMA technique, i.e., beams of the type described in FIG. 8 may be used,and users are separated within a given cell and among adjacent cellsthrough the use of codes.

FIG. 11 illustrates an OFDM technique in accordance with the invention.In this technique, duplexing between the uplink and downlink portions isperformed adaptively in the frequency domain, using orthogonal frequencytones, rather than the conventional FDD as described in conjunction withFIG. 7. This technique allows for asymmetric uplink and downlinkcapacity. As shown in FIG. 11, a downlink portion 102 and an uplinkportion 104 are separated in frequency by a variable boundary 106. Thereare a total of M orthogonal frequency tones 110 in the band of interest.In the FIG. 11 example, tones 1 through k are assigned to the uplinkportion 104, while tones k+1 to M are assigned to the downlink portion102. Unlike the conventional FDD technique, this OFDM technique allowsfrequencies to be assigned adaptively between uplink and downlink inorder to accommodate variations in demand. Within a given cell, uplinkand downlink portions may be separated, e.g., in the discrete Fouriertransform (DFT) domain based on assignment of OFDM carriers. Userswithin a given beam can be separated, e.g., by using different timeslots or different codes, or other suitable techniques. Users separationamong different beams of a given cell may be implemented using differentcodes. Among adjacent cells, frequencies or codes may be used toseparate the various users.

In the OFDM technique of FIG. 11, appropriate timing synchronization isgenerally required between the base station and the subscriber unit inorder to maintain tone orthogonality. This timing synchronization can beeasily achieved through a “sync” control channel transmitted by the basestation to the subscriber unit. Frequency synchronization is alsogenerally required between the base station and the subscriber unit.Since the subscriber unit in the illustrative embodiment is fixed, thereis no frequency offset due to Doppler effects. Hence, frequencysynchronization in such a system can be implemented in a straightforward manner. Accurate power control is also generally requiredbetween the base station and the subscriber unit. Again, since thesubscriber unit is fixed, the time variation of the wireless channel isvery slow, which allows for straight forward implementation of accuratepower control.

FIG. 12 shows a downlink, i.e., base-to-subscriber, transmitter 120 inaccordance with the invention, suitable for use with the OFDM techniqueof FIG. 11. The transmitter 120 includes an inverse DFT (IDFT) orinverse fast Fourier transform (IFFT) element 124, a parallel-to-serialconverter 126, and multipliers 128, 130 and 132. The M orthogonalfrequency tones are applied to the IDFT or IFFT element 124. The first kof the M tones, which are assigned to the uplink portion 104, contain nodata, e.g., all zero levels. Tones k+1 to M, which are assigned to thedownlink portion 102, contain the downlink data, e.g., +1 and −1 levels.The element 124 generates the inverse transform of the M applied tones,and its output is supplied to the parallel-to-serial converter 126. Theserial output of converter 126 is supplied to multiplier 128 in whichthe serial output is multiplied by a user-specific spreading code. Themultiplier 128 is shown in a dashed box to indicate that it is anoptional element. It presence will depend on whether the users in a beamare separated using codes, i.e., multiplier 128 will be present if theusers in a beam are separated using codes. The output of the multiplier128 is then multiplied by a sector-specific spreading code in multiplier130, and the resulting signal is modulated onto a carrier correspondingto frequency ω₀ in multiplier 132. The output of multiplier 132 is adownlink signal which is transmitted from the base station to asubscriber unit.

FIG. 13 shows a corresponding downlink receiver 140 which may beimplemented in the subscriber unit. The receiver 140 demodulates thereceived downlink signal using multiplier 142, and the demodulatedsignal is low-pass filtered using integrator 144. The filtered signal isde-spread by multiplying it by the sector-specific spreading code inmultiplier 146, and summing in a sum element 148. If necessary, i.e., ifthe users in a beam are separated using codes, the output of sum element148 is multiplied by the user-specific spreading code in multiplier 150and then summed in a sum element 152. Otherwise, the elements 150, 152may be eliminated and the output of sum element 148 is applied directlyto a serial-to-parallel converter 154. The parallel outputs of theconverter 154 are applied to a DFT or FFT element 156, which performs aDFT or FFT operation to recover the M tones. The first k tones, assignedto the uplink, do not include downlink data and are therefore discarded.The downlink data is present on tones k+1 to M.

FIGS. 14 and 15 show an uplink, i.e., subscriber-to-base, transmitterand an uplink receiver, respectively, for implementing the OFDMtechnique of FIG. 11. The uplink transmitter 220 of FIG. 14 includes anIDFT or IFFT element 224, a parallel-to-serial converter 226, anoptional user-specific spreading code multiplier 228, a sector-specificspreading code multiplier 230, and a multiplier 232 for modulating thedownlink signal onto a carrier. These elements operate in substantiallythe same manner as the corresponding elements of the downlinktransmitter 120 of FIG. 12, but the uplink data is applied to the firstk tones, while tones k+1 through M contain no data. The output ofmultiplier 232 is an uplink signal which is transmitted from asubscriber unit to a base station. FIG. 15 shows the correspondinguplink receiver 240 which may be implemented in a base station. Thereceiver 240 includes a demodulating multiplier 242, an integrator 244,a sector-specific spreading code multiplier 246 and associated sumelement 248, an optional user-specific spreading code multiplier 250 andits associated sum element 252, a serial-to-parallel converter 254, anda DFT or FFT element 256. These elements operate in substantially thesame manner as the corresponding elements of the downlink receiver 140of FIG. 13, but the uplink data is present on the first k tones, whilethe tones k+1 through M do not include uplink data and are discarded.

FIGS. 16 and 17 show a multi-code CDMA transmitter 300 and a multi-codeCDMA receiver 400, respectively, in accordance with the invention. Thetransmitter 300 and receiver 400 are suitable for use with, e.g., theabove-described CDD time-slotted CDMA and TDD time-slotted CDMAtechniques of the invention. In the transmitter 300 and receiver 400, itis assumed that there are a total of N spreading codes per beam in agiven sector or cell of the system. The transmitter 300 receives N inputsignals in corresponding beam-specific code multipliers 302-i, i=1, 2, .. . N. The outputs of the multipliers 302-i are summed in element 304,and then multiplied by a sector-specific spreading code in multiplier306. The output of multiplier 306 is modulated onto a carriercorresponding to frequency ω₀ in multiplier 308. The resulting outputsignal may be transmitted from a base station to one or more subscriberunits.

The multi-code CDMA receiver 400 receives an input signal which isdemodulated in multiplier 402, low-pass filtered in integrator 404, andthen de-spread using the sector-specific spreading code in a multiplier406 and associated sum element 408. A sampling switch 410 is controlledso as to “dump” samples every symbol time. The samples are de-spread inmultipliers 412-i, i=1, 2, . . . N, and associated sum elements 414-i,using corresponding beam-specific codes. Sampling switches 416-i delivera separate output for each of the beam-specific codes. The receiver 400may be implemented in a base station to process signals received frommultiple subscriber units of the system.

It should be emphasized that the exemplary wireless systems and devicesdescribed herein are intended to illustrate the operation of theinvention, and therefore should not be construed as limiting theinvention to any particular embodiment or group of embodiments. Forexample, although well suited for implementation in an omni-beam ornarrow-beam FWL system, the invention can be used in other applications.In addition, a system in accordance with the invention may includeadditional elements, such as, for example, mobile switching centers(MSCs) for connecting one of more of the base stations to a publicswitched telephone network (PSTN), and a memory for storing, e.g., userdata and billing information. Furthermore, it will be apparent to thoseskilled in the art that the transmitters and receivers shown herein forpurposes of illustrating the invention may be implemented in manydifferent ways, and may include a number of additional elements, e.g.,diplexers, downconverters, upconverters, signal sources, filters,demodulators, modulators, baseband signal processors, etc., configuredin a conventional manner. These and numerous other alternativeembodiments within the scope of the following claims will therefore beapparent to those skilled in the art.

What is claimed is:
 1. A method of communicating information in awireless cellular communication system, the method comprising the stepof: communicating information between a plurality of subscriber units ofthe system and a base station in a cell of the system over at least oneof an uplink and a downlink, wherein communications on the uplink areseparated from communications on the downlink using time divisionduplexing; wherein communications between the base station and at leasta subset of the plurality of subscriber units in the cell are separatedusing a code division multiple access technique; and wherein each of thesubscriber units within a given antenna beam associated with a givensector of the base station is assigned a distinct beam-specificspreading code, the beam-specific spreading code being used inconjunction with a sector-specific spreading code to separate thecommunications between the base station and the subscriber units withinthe given antenna beam in accordance with the code division multipleaccess technique.
 2. The method of claim 1 wherein the system is a fixedwireless loop system.
 3. The method of claim 1 wherein the code divisionmultiple access technique is time-slotted code division multiple access.4. The method of claim 1 wherein the given antenna beam comprises anelectronically-steered beam at a particular point in time, and has awidth sufficient to provide simultaneous coverage for at least n of theplurality of subscriber units at that time, where n is greater than orequal to two.
 5. The method of claim 1 wherein the time divisionduplexing further includes the step of assigning a first subset of a setof time slots to the uplink and a second subset of the set of time slotsto the downlink.
 6. The method of claim 5 further including the step ofvarying the assignment of time slots to uplink and downlink inaccordance with uplink and downlink traffic demands.
 7. An apparatus forcommunicating information in a wireless communication system, theapparatus comprising: a base station operative to communicate with aplurality of subscriber units in a cell of the system over at least oneof an uplink and a downlink, wherein communications on the uplink areseparated from communications on the downlink using time divisionduplexing, and communications between the base station and at least asubset of the plurality of subscriber units in the cell are separatedusing a code division multiple access technique; wherein each of thesubscriber units within a given antenna beam associated with a givensector of the base station is assigned a distinct beam-specificspreading code, the beam-specific spreading code being used inconjunction with a sector-specific spreading code to separate thecommunications between the base station and the subscriber units withinthe given antenna beam in accordance with the code division multipleaccess technique.
 8. The apparatus of claim 7 wherein the system is afixed wireless loop system.
 9. The apparatus of claim 7 wherein the codedivision multiple access technique is time-slotted code divisionmultiple access.
 10. The apparatus of claim 7 wherein the given antennabeam comprises an electronically-steered beam at a particular point intime, and has a width sufficient to provide simultaneous coverage for atleast n of the plurality of subscriber units at that time, where n isgreater than or equal to two.
 11. The apparatus of claim 7 wherein thebase station is further operative to assign a first subset of a set oftime slots to the uplink and a second subset of the set of time slots tothe downlink.
 12. The apparatus of claim 11 wherein the base station isfurther operative to vary the assignment of time slots to uplink anddownlink in accordance with uplink and downlink traffic demands.
 13. Anapparatus for communicating information in a wireless communicationsystem, the apparatus comprising: a first subscriber unit operative tocommunicate with a base station in a cell of the system over at leastone of an uplink and a downlink, wherein communications on the uplinkare separated from communications on the downlink using time divisionduplexing, and communications between the base station and at least oneother subscriber unit in the cell are separated from communicationsbetween the base station and the first subscriber unit using a codedivision multiple access technique; wherein each of the subscriber unitswithin a given antenna beam associated with a given sector of the basestation is assigned a distinct beam-specific spreading code, thebeam-specific spreading code being used in conjunction with asector-specific spreading code to separate the communications betweenthe base station and the subscriber units within the given antenna beamin accordance with the code division multiple access technique.
 14. Theapparatus of claim 13 wherein the system is a fixed wireless loopsystem.
 15. The apparatus of claim 13 wherein the code division multipleaccess technique is time-slotted code division multiple access.
 16. Theapparatus of claim 13 wherein the given antenna beam comprises anelectronically-steered beam at a particular point in time, and has awidth sufficient to provide simultaneous coverage for at least nsubscriber units at that time, where n is greater than or equal to two.17. The apparatus of claim 13 wherein the time division duplexingassigns a first subset of a set of time slots to the uplink and a secondsubset of the set of time slots to the downlink.
 18. The apparatus ofclaim 17 wherein the assignment of codes to uplink and downlink isrepeated for each of a plurality of time slots, such that the number ofcodes in the first and second subsets varies in accordance with uplinkand downlink traffic demands.